Melt butter in a
large saucepan with the onions and garlic, and cook on low-medium heat until they're soft and translucent. Whisk in the flour and cook on low-medium heat (don't let it burn) for 2 minutes. Add liquid
slowly and keep whisking as the sauce thickens. Keep on low so the sauce doesn't stick and burn as you finish the recipe. Add more liquid if needed to keep it from getting
too thick.